Broadband high frequency balun

ABSTRACT

A broadband high frequency balun is connectable between balanced and unbalanced transmission lines so that these lines are essentially colinear. The balun comprises a coaxial cable and a shorted stub in approximately a half-loop configuration, the balanced line being connected to the cable and stub within a conductive housing or shield. The cable-stub spacing is substantially greater than the effective length of the stub, thereby decreasing the lower frequency operating limit of the balun. The addition of a lossy layer to the inner surface of the housing permits a substantial increase in the operating bandwidth of the balun by suppressing adverse resonance effects within the housing.

Mite States Epis et al.

i atent I 1 Jan. 15, 1974 BROADBAND HIGH FREQUENCY BALUN [75] Inventors:James J. Epis, Sunnyvale; Samuel Chung-shu Kuo, Cupertino, both ofCalif.

21 Appl. No.: 314,835

[52] US. Cl. 333/26, 333/33 [51] Int. Cl. 1103b 7/38, H03h 7/42 [58]Field of Search 333/25, 26, 32, 33;

[56] References Cited UNlTED STATES PATENTS 2,581,156 l/l952 Weighton333/26 X 2,530,048 ll/l950 Driscoll 333/26 X FOREIGN PATENTS ORAPPLICATIONS 877,342 12/1942 France 333/26 OTHER PUBLICATIONS Fubini etaL-A Wide-Band Transformer from an Unbalanced to a Balanced Line inProceedings of the IRE-Waves and Electrons Section October, 1947; pages1,l53l,l55.

MacKenzie-Some Recent Advances in Coaxial Components for Sweep FrequencyInstrumentation in the Microwave Journal June 1969; pages 7374.

Primary Examiner-Rudolph V. Rolinec Assistant ExaminerMarvin NussbaumAtt0rneyJohn F. Lawler [57] ABSTRACT A broadband high frequency balun isconnectable between balanced and unbalanced transmission lines so thatthese lines are essentially colinear. The balun comprises a coaxialcable and a shorted stub in approximately a half-loop configuration, thebalanced line being connected to the cable and stub within a conductivehousing or shield. The cable-stub spacing is substantially greater thanthe effective length of the stub, thereby decreasing the lower frequencyoperating limit of the balun. The addition of a lossy layer to the innersurface of the housing permits a substantial increase in the operatingbandwidth of the balun by suppressing adverse resonance effects withinthe housmg.

4 Claims, 5 Drawing Figures BROADBAND HIGH FREQUENCY BALUN BACKGROUND OFTHE INVENTION This invention relates to baluns and more particularly toan improved balun capable of operating at high frequencies.

A balun is a device which effectually transforms a TEM-mode wavepropagating on a balanced twoconductor transmission line into anotherTEM-mode wave propagating inside an unbalanced-type transmission line,the latter typically being a coaxial line. The TEM-mode transformationis reciprocal.

There are many applications for such a device. An important applicationis the connection of a coaxialline output or input of a transmitter or areceiver to any type of antenna that can be excited properly only bymeans of a balanced two-conductor transmission line. In many instancesit is desired if not required that the balun interconnecting the twodifferent types of transmission lines be capable of operatingeffectively and efficiently over broad frequency bands, often over verybroad bands. An everincreasing demand for very broadband conical andcavity-backed spiral antennas for operation up to 30 to 40 GI-Iz existat the present time. Utilization of a broadband balun provides the mosteconomical convenient means to achieve proper excitation of theseantennas. The provision of a satisfactory reasonably efficient verybroadband balun for these important newly developing applications forspiral antennas is a principal objective of this invention.

A prior art balun useful at high microwave frequencies is described inan article entitled A Wide-Band Balun by McLaughlin et al. in IRETransaction on Microwave Theory and Techniques, July 1958, at pages314-316. The upper limit of useful frequency range for this balun isabout 18 GHz and furthermore the input and output lines to this balunare spatially orthogonal. Accordingly, this balun cannot be used overthe full range of a spiral antenna, for example, operating over a bandof 1.3 to 40 I-IHz. Furthermore, colinear feed arrangements cannot beaccommodated by this balun.

OBJECTS AND SUMMARY OF INVENTION An object of this invention is theprovision of a balun having insertion loss and input VSWR performancescomparable to state-of-the-art baluns but having extremely broadbandwidths, i.e., 36:1.

A further object is to provide a balun of this type for use atfrequencies up to 40 GI-lz.

Another object is to provide a balun in which the unbalanced coaxialinput transmission line is colinear or nearly colinear with the balancedline.

These and other objects of the invention are achieved with a balunfeaturing a cable and shorted stub spaced apart by a distance greaterthan the length of the stub and housed in a conductive shield. The cableand stub are configured to form approximately a half loop and thebalanced line connected to the cable and stub extends in a directionparallel to the cable. The band width of the balun is greatly increasedwith a slight increase in insertion loss through suppression ofresonances of the TEM-wave, TE-wave and TM-w-ave modes within thecavityby disposition ofa lossy material in the housing.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a balunembodying the invention;

FIG. 2 is a section taken on line 22 of FIG. 1;

FIG. 3 is a greatly enlarged sectional view of the junction of thebalanced line with the coaxial line and shorted stub;

FIG. 4 is a view taken on line 4-4 of FIG. 3; and

FIG. 5 is a section taken on line 55 of FIG. 4.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, abalun embodying the invention is shown at 10 and comprises a cylindricalhousing 11 having an axis A, a side wall 12 and end walls 13 and 14 atopposite ends of the side wall. The housing is preferably made ofconductive material such as copper or brass and defines a cavity 16within which energy from a balanced line 18 is transformed to anunbalanced line 19 (or vice versa). A central opening 21 in end wall 13permits the balanced line to extend into the cavity without makingelectrical contact with the housing. A standard connector 22 attached toend wall 14 permits connection to the balun of the external unbalancedline 19, shown as a coaxial line.

Extending into the housing from end wall 14 at connector 22 is aninverted L-shaped coaxial cable 24 having a first leg 25 extendingparallel to housing axis A and a second leg 26 extending radiallyinwardly from and substantially at right angles to leg 25 for connectionto balanced line 13. Cable 24 has an inner conductor 24a and an outerconductor 24b, the latter being connected to end wall 14. On thediametrically opposite side of the cavity from cable 24 is a similarlyinverted L-shaped conductive stub 28 having a first leg 29 electricallyconnected to and extending inwardly from the end wall 14 and a secondleg 30 extending radially inwardly and substantially at right angles tofirst leg 29. Cable 24 and stub 28 lie in a plane containing the axis Aof the housing and are symmetrically disposed about the axis in theshape of a half rectangular loop as shown in FIG. 2.

Balanced line 18 comprises a pair of conductors 32 and 33 which, in theembodiment shown, are formed or deposited as thin films on a low lossdielectric strip 35. This balanced line extends from its connection tocable 24 and stub 28 within housing 11 to utilization apparatus, notshown, such as a spiral antenna.

The connection of the balanced line 18 to the cable and stub is shown inFIG. 3. Cable outer conductor 24b at the inner end of radial leg 26 iselectrically connected to conductor 32 of balanced line 18. Innerconductor 24a extends through an opening 37 in and therefore iselectrically insulated from conductor 32 and passes through insulatorstrip 35 for electrical contact with conductor 33 and stub leg 30. Inpractice, stub 28 preferably is tubular in shape and may then have anapertured plug 39 press-fitted into the inner end of leg 30 forreceiving the extension of the inner conductor as shown. The plug, innerconductor, and stub leg 30 are electrically connected to conductor 33 ofthe balanced line by solder 40 or the like. Optimum operation of thebalun is achieved by forming the outer surfaces of coaxial cable 24 andstub 28 such that those surfaces are virtually identical.

In prior art baluns of the general type described above, the distance dbetween component parts corresponding to legs 25 and 29 of the cable andstub, respectively, generally determine the highest usable frequency ofthe devices. More particularly, as the distance d approaches 0.2 A where)t is the operating wavelength, currents on the exterior of legs 25 and29 begin to radiate.

Without the housing 11 functioning as an electromagnetic shield aroundthose legs, such radiation would render the balun of the presentinvention useless for its intended purpose. More specifically, theshielding effect of housing 11 prevents such radiation, therebyextending the frequency range of the device. As the operating frequencyis increased, however, resonant cavity effects of housing 11 come intoplay. With such increase in frequency, the cavity in the housing becomeselectrically large enough in diameter to support waveguide-type modes.These waveguide-type modes are TE- and TM-modes as distinguished fromTEM-modes. The currents on the legs of the stub and coaxial cable withinthe cavity excite such modes. The effect of the waveguide-type modes inthe cavity is to cause insertion loss spikes periodically across theoperating band. In order to eliminate these spikes, and thereby greatlyincrease the operating bandwidth, a layer or cylinder 42 of dissipativeor lossy material is disposed adjacent to the side wall of the cavity,as shown in FIGS. 1 and 2. This material suppresses these waveguide-typemodes and eliminates the insertion loss spikes caused by them while atthe same time producing an acceptably small increase in the averageinsertion loss of the device across the band.

It should be noted that the balun described above without the lossymaterial 42 and in which the distance d is greater than the height h ofthe stub and cable provided satisfactory performance as a balun over a:1 bandwidth, the insertion loss being less than 1.3 db. Thus, forapplications having this or a smaller bandwidth requirement, the lossymaterial may be omitted, with the advantage of a decrease in insertionloss. Details are described below.

A shielded half-loop balun of the type described above without lossyliner 42 was constructed and successfully operated and had the followingdimensions and operating characteristics:

Cavity Inner diameter 2.75 inches Length (axial) 1.25 inches LoopDistance d 1.5 inches Height Ii 0.697 inches Diameter of cable/stub0.085 inches Balanced line Thickness! (gap) 0.031 inches Characteristicimpedance 62 ohms Bandwidth 0.256 GHZ to 3.84 GHZ (15.011) Maximuminsertion loss 1.2 db

The addition to the above-described tested balun of a complete cylinder42 of 0.375 inch thick lossy maerial made of carbonized foam by EmersonCummings, Inc. and designated as AN-73, adjacent to the cylindrical sidewall 12 increased the useful bandwidth from the 0.256 GHZ -3.84 GI-Iz(15:1) range to 0.269 GHz to 9.71 61-12 (36:1) while maintaining theinsertion loss less than 1.5 db across that band. In addition tosuppressing TE- and TM-mode resonances, the lossy material alsosuppressed TEM-mode resonances which occurred in the cavity as aconsequence of the effective electrical length of stub leg 29approaching A2 and 1.0 A.

The higher frequency versions of baluns which embody this invention areachieved by scaling the dimensions of the balun components in accordancewith the frequency desired or required. Such scaling is demonstrated inTable I for baluns without lossy cylinder 42, beginning with the testedmodel described earlier.

TABLE I Semi-Rigid Coaxial Line Frequency Band Comment UT 85 0.256 to3.84 GHZ Tested Model (15:1 Bandwidth) UT 0.311 to 4.663 GHz 70/85 ScaleModel UT 47 0.462 to 6.945 0112 47/85 Scale Model UT 35 0.622 to 9.326GHz 35/85 Scale Model UT 20 1.088 to 16.32 6112 20/85 Scale Model All ofthe coaxial cables referenced in the table and satisfactory connectorsfor them are commercially available items. Dimensioning of the balancedline is readily and accurately controlled by photo-etching the lines ona dielectric strip of properly scaled dimensions. Finally, the housing11 is machined so that it is readily constructed accurately to theprecise scaled dimensions. While Table I demonstrates how the preferredembodiment of the invention without lossy cylinder 42 is scaled for useat higher frequencies, it does not necessarily follow that directlyscaled models are optimum designs.

Table II illustrates the effect of scaling the dimensions of theforegoing tested embodiment of the invention which included the lossylayer 42 within the cavity to greatly expand the operating bandwidth ofthe balun.

TABLE II Semi-Rigid Coaxial Line Frequency Range Comment UT 0.269 to9.71 GHz Tested Model (36:1 Bandwidth) UT 70 0.3266 to 11.79 GHZ UT 470.4865 to 17.57 GHZ UT 35 0.6533 to 23.58 GHz UT 20 1.143 to 41.26 GHzThe last version of the balun listed in this table has an upperoperating frequency limit in excess of 40 6112.

What is claimed is: 1. A device for transforming an unbalancedtransmission line to a balanced transmission line comprising anelectrically conductive casing having a side wall and parallel end wallsconnected to opposite ends of the side wall and defining a cavitytherewithin,

one of said end walls having an opening through which the balanced lineextends into the cavity transversely of and insulated from said one endwall,

said unbalanced line comprising a coaxial cable projecting into thecavity from the other end wall and having an outer conductor connectedat opposite ends to said other end wall and to one conductor of saidbalanced transmission line, respectively, said cable also having aninner conductor connected within the cavity to the other conductor ofsaid balanced line,

a conductive stub electrically connecting the other conductor of saidbalanced line to said other end wall,

said stub and said cable having substantially equal outer diameters andsubstantially equal lengths, and having first parallel portions,respectively, projecting into the cavity from said other end wall andbeing spaced apart by a distance substantially greater than the lengthof each of said first portions.

2. The device according to claim 1 with a thin layer of lossy materialon the interior of said side wall.

3. A balun for interconnecting an unbalanced transmission line with abalanced transmission line comprisa cylindrical conductive housinghaving an axis and axially spaced end walls,

one of said end walls having a central opening therein through whichsaid balanced line extends axially into and insulated from said housing,

an L-shaped coaxial cable having a first leg extending into the housingfrom the other end wall parallel to and offset from said axis and havinga second leg extending radially inwardly from the first leg toward saidbalanced line,

said cable having an inner conductor and an outer conductor, said outerconductor being electrically connected to said housing, means forelectrically connecting said unbalanced line on the outside of saidhousing to said first leg of the cable whereby the unbalanced andbalanced lines extend substantially parallel to one another, an L-shapedconductive stub in said housing having a first leg connected to saidother end wall and extending parallel to and offset from said axis, saidstub also having a second leg connected to and extending radiallyinwardly toward said balanced line colinearly with the second leg of thecoaxial cable, said balanced line having first and second conductorsconnected to the outer and inner conductors, respectively, of saidcable, said second conductor also being connected to said second leg ofsaid stub, the spacing between the first legs of said cable and saidstub being substantially greater than the length of said first leg ofthe stub. 4. The balun according to claim 3 in which said housing has aside wall, and a resistive coating on the inner surface of said sidewall.

1. A device for transforming an unbalanced transmission line to abalanced transmission line comprising an electrically conductive casinghaving a side wall and parallel end walls connected to opposite ends ofthe side wall and defining a cavity therewithin, one of said end wallshaving an opening through which the balanced line extends into thecavity transversely of and insulated from said one end wall, saidunbalanced line comprising a coaxial cable projecting into the cavityfrom the other end wall and having an outer conductor connected atopposite ends to said other end wall and to one conductor of saidbalanced transmission line, respectively, said cable also having aninner conductor connected within the cavity to the other conductor ofsaid balanced line, a conductive stub electrically connecting the otherconductor of said balanced line to said other end wall, said stub andsaid cable having substantially equal outer diameters and substantiallyequal lengths, and having first parallel portions, respectively,projecting into the cavity from said other end wall and being spacedapart by a distance substantially greater than the length of each ofsaid first portions.
 2. The device according to claim 1 with a thinlayer of lossy material on the interior of said side wall.
 3. A balunfor interconnecting an unbalanced transmission line with a balancedtransmission line comprising a cylindrical conductive housing having anaxis and axially spaced end walls, one of said end walls having acentral opening therein through which said balanced line extends axiallyinto and insulated from said housing, an L-shaped coaxial cable having afirst leg extending into the housing from the other end wall parallel toand offset from said axis and having a second leg extending radiallyinwardly from the first leg toward said balanced line, said cable havingan inner conductor and an outer conductor, said outer conductor beingelectrically connecTed to said housing, means for electricallyconnecting said unbalanced line on the outside of said housing to saidfirst leg of the cable whereby the unbalanced and balanced lines extendsubstantially parallel to one another, an L-shaped conductive stub insaid housing having a first leg connected to said other end wall andextending parallel to and offset from said axis, said stub also having asecond leg connected to and extending radially inwardly toward saidbalanced line colinearly with the second leg of the coaxial cable, saidbalanced line having first and second conductors connected to the outerand inner conductors, respectively, of said cable, said second conductoralso being connected to said second leg of said stub, the spacingbetween the first legs of said cable and said stub being substantiallygreater than the length of said first leg of the stub.
 4. The balunaccording to claim 3 in which said housing has a side wall, and aresistive coating on the inner surface of said side wall.